The explosion of new wireless technologies and applications demands prudent use of the limited, usable radio spectrum. This has fostered regulatory changes to move from the current fixed spectrum allocation policies to flexible spectrum usage models. In the USA, the Federal Communications Commission (FCC) which is charged with regulating interstate and international communications by radio, television, wire, satellite and cable, is for instance taking steps towards adopting policies permitting the use of low-power portable devices in the VHF-UHF spectrum. Regulatory steps are also moving in a direction of permitting more secondary spectrum available for medical devices and body sensor networks. Interest has also been growing in the UK and other EU counties to adopt innovative spectrum sharing models where licensed bands allocated to primary systems are opened for secondary usage.
One of the requirements for operation on secondary spectrum basis is that the secondary system transmission does not cause any harmful interference to the primary systems. This requirement is typically met by spectrum sensing and operating on spectral regions where primary systems are not found to be active. Spectrum sensing may involve detection of the presence of transmissions from primary systems.
The benefits of cooperation and relaying in wireless systems are widely recognized, and also part of upcoming standards (e.g., IEEE 802.15 WPAN Task Group on Body Area Networks). These techniques are known to lead to better system performance and used for example for range extension, improving error rates etc. Cognitive wireless systems based on cooperative relaying will thus be of natural interest in such future wireless standards.
A cognitive network where two source-destination links, a primary link and a secondary link, share the same spectral resource has been recently investigated in an information theoretic study presented in N. Devroye, P. Mitran and V. Tarokh, “Achievable rates in cognitive radio,” IEEE Trans. Inform. Theory, vol. 52, no. 5, pp. 1813-1827, May 2006, and A. Jovicic and P. Viswanath, “Cognitive radio: an information-theoretic perspective”. In these references, a cognitive transmitter is assumed to have perfect prior information about the signal transmitted by a primary transmitter (see also P. Mitran, N. Devroye and V. Tarokh, “On compound channels with side information at the transmitter,” IEEE Trans. Inform. Theory, vol. 52, no. 4, pp. 1745-1755, April 2006, the entire content of which is incorporated herein by reference). However, imperfect information on the radio environment (e.g., on the primary activity) at the cognitive transmitter (or node) is expected to be a major impediment to the implementation of the cognitive principle, as described in more detail in A. Sahai, N. Hoven and R. Tandra, “Some fundamental limits on cognitive radio,” in Proc. Allerton Conference on Communication, Control, and Computing, October 2004. Moreover, traffic dynamics at the primary are of great importance in defining the performance of cognitive radio, but random packet arrival cannot be easily incorporated in a purely information theoretic analysis.
FIG. 1 shows a schematic architecture of a cognitive wireless relay system topology, where a cognitive system transmitter (CTx) 10 transmits data to a cognitive system receiver (CRx) 20 with the aid of a cognitive system relay (CR) 30. This cognitive system operates on secondary sharing basis on a certain portion of spectrum that is licensed to certain primary systems which include a primary system transmitter 40.
FIG. 2 shows a schematic timing schedule of a naïve sensing and communication protocol which may be employed in a system according to FIG. 1.
Sensing is performed by the CTx 10 periodically in steps 100, 200, and so on. If no primary system transmissions are present, transmission is done in two phases of respective successive transmission periods TP1, TP2, and so on. In the first phase (steps 110, 210, . . . ), the CTx 10 transmits data to the CRx 20, which is also received at CR 30. The CR 30 performs some signal processing on the received data (for e.g., amplification, decoding, etc.) and forwards or broadcasts it to the CRx 10 in the second phase (steps 120, 220, . . . ).
However, periodic sensing is required to monitor the presence/reappearance of primary signal transmissions. As can be seen from FIG. 2, this periodic sensing in steps 100, 200, . . . is an overhead and reduces information throughput significantly. It also has an implication on latency requirements and can be a critical issue in delay-sensitive applications.